Iron is an essential trace element that is required for growth and development of all living organisms; it is indispensable for DNA synthesis and a broad range of metabolic processes. However, disturbances of iron metabolism have been implicated in a number of significant mammalian diseases, including, but not limited to iron deficiency anemia, hemosiderosis or the iron overload disease hemochromatosis (Andrews, N. C. (2000) Annu. Rev. Genomics Hum. Genet. 1, 75-98; Philpott, C. C. (2002) Hepatology 35, 993-1001; Beutler et al., (2001) Drug-Metab. Dispos. 29, 495-499). Iron content in mammals is regulated by controlling absorption predominantly in the duodenum and upper jejunum, and is the only mechanism by which iron stores are physiologically controlled (Philpott, C. C. (2002) Hepatology 35, 993-1001). Following absorption, iron is bound to circulating transferrin and delivered to tissues throughout the body. The liver is the major site of iron storage. There, transferring-bound iron is taken into the hepatocytes by receptor-mediated endocytosis via the classical transferrin receptor (TfR1) (Collawn et al., (1990) Cell 63, 1061-1072) and presumably in greater amounts via the recently identified homologous transferrin receptor 2 (TfR2) (Kawabata et al., (1999) J. Biol. Chem. 274, 20826-20832). The extracellular domain of this protein is 45% identical to the corresponding portion of TfR1 (Id.). TfR2 can also bind diferric transferrin and facilitate the uptake of iron. Mutations in TfR2 have been associated with certain forms of hemochromatosis demonstrating the important role for TfR2 in iron homeostasis (Philpott, C. C. (2002) Hepatology 35, 993-1001; Camasehella et al., (2000) Nat. Genet. 25, 14-15; Fleming et al., (2002) Proc. Natl. Acad. Sci. USA 99, 10653-10658). TfR2 is predominantly expressed in the liver (Fleming et al., (2000) Proc. Natl. Acadi. Sci. USA 97, 2214-2219; Subramaniam et al., (2002) Cell Biochem. Biophys. 36, 235-239), however, the exact cellular localization is still unknown.
A feedback mechanism exists that enhances iron absorption in individuals who are iron deficient, and reduces iron absorption in subjects with iron overload (Andrews, N. C. (2000) Annu. Rev. Genomics Hum. Genet. 1, 75-98; Philpott, C. C. (2002) Hepatology 35, 993-1001; Beutler et al., (2001) Drug-Metab. Dispos. 29, 495-499). Nonetheless, the molecular mechanism by which the intestine responds to alterations in body iron requirements remains poorly understood. In this context, hepcidin, a recently identified mammalian polypeptide (Krause et al., (2000) FEBS Lett. 480, 147-150; Park et al., (2001) J. Biol. Chem. 276, 7806-7810), is predicted as a key signaling component regulating iron homeostasis (Philpott, C. C. (2002) Hepatology 35, 993-1001; Nicolas et al., (2002) Proc. Natl. Acad. Sci. USA 99, 4396-4601). Hepcidin was initially isolated as a 25 amino acid (aa) polypeptide in human plasma and urine exhibiting antimicrobial activity (Krause et al., (2000) FEBS Lett. 480, 147-150; Park et al., (2001) J. Biol. Chem. 276, 7806-7810). A hepcidin cDNA encoding an 83 aa precursor in mice and an 84 aa precursor in rat and man, including a putative 24 aa signal peptide, were subsequently identified searching for liver specific genes that were regulated by iron (Pigeon et al., (2001) J. Biol. Chem. 276, 7811-7819).
Since the discovery that hepcidin expression is abolished in mice exhibiting iron-overload due to the targeted disruption of upstream stimulatory factor 2 (Usf2) gene resembling the same phenotype as found in Nicolas, O., Bennoun, M., Devaux, I., Beaumont, C., Grandchamp, B., Kahn, A. & Vaulont, S. (2001) Proc. Natl. Acad. Sci. USA 98, 8780-8785, it has become evident that this peptide plays a pivotal role in iron metabolism. In contrast, overexpression of hepcidin was shown to result in severe iron deficiency anemia in transgenic mice (Nicolas et al., (2002) Proc. Natl. Acad. Sci. USA 99, 4396-4601), indicating that hepcidin is a central regulator of iron homeostasis. However, the mechanism by which hepcidin balances the body iron stores or adjusts the dietary iron absorption still remains to be identified. In this respect, the cellular and subcellular localization of this peptide is of decisive importance in the search for the signaling route. Although Northern blot analysis of human and mouse hepcidin mRNA levels in various organs revealed that hepcidin is predominantly expressed in liver, no data exist on the cellular source of this polypeptide (Krause et al., (2000) FEBS Lett. 480, 147-150; Park et al., (2001) J. Biol. Chem. 276, 7806-7810; Nicolas et al., (2002) Proc. Natl. Acad. Sci. USA 99, 4396-4601).